1. Field of the Invention
The present invention relates to a filter apparatus for use with a resin material extruder to filter dust and other foreign objects from a molten resin material. More particularly, the invention relates to a screen changer, or a device for changing the screens in the filter apparatus.
2. Related Art
Resin materials which are being melted and kneaded in and extruded from extruders are in many cases contaminated by dust and other solid unmeltable matter. In order to remove such contaminants, extruders are equipped with a filter apparatus which has screens provided in the passageway of the molten material from the extruder such that any solid contaminants are filtered off to produce a clean melt which is supplied into a die and other subsequent devices and components. However, as the filter is used, the contaminants removed by filtration collect on the surface of the screens on the upstream side and reduce the effective area of filtration such that the resistance to the flow of the molten material is increased, thereby lowering the operating efficiency of the extruder. To avoid this problem, the screens are cleaned to remove the collecting contaminants or replaced by new screens either periodically or as the need arises.
A representative type of the conventional screen changers is illustrated in FIGS. 4 to 11. The screen changer 1 in FIG. 4 comprises a housing 2 with two parallel through-holes 23 and 24 having a circular cross section normal to its axis, two cylindrical slide bars 3 and 4 that are fitted into said through-holes 23 and 24, respectively, in a liquid-tight and reciprocating manner, and hydraulic cylinders 5 and 6 that are fixed to said housing 2 via a plurality of tie-bars 8, nuts 8a and a clamping plate 7 and which have piston rods 5a and 6a connected to said slide bars 3 and 4, respectively.
As shown in FIG. 5, said housing 2 has a through-channel 11 formed in a direction perpendicular to said through-holes 23 and 24, said through-channel 11 has an inflow channel 11a which separates into two branches and the two branches, after crossing the respective through-holes 23 and 24, meet part of the way in an outflow channel 11b.
Each of the slide bars 3 and 4 has a channel portion and a filter portion that each communicate with said through-channel 11 and which penetrate it in a direction normal to its axis. Said channel and filter portions are composed of upstream compartments 31 and 41, the screen 9, a breaker plate 10 and downstream compartments 32 and 42. The upstream compartments 31 and 41 are formed such that openings of sufficiently large cross sections are provided to increase the effective area of filtration, or the two-dimensional area of the screen 9. On the other hand, the downstream compartments 32 and 42 are formed such that the size of the cross sections of their openings will decrease smoothly in a downward direction to insure that the large cross sections of the openings just after the breaker plate 10 will match the small cross sections of the openings in the outflow channel 11b. The through-holes 23 and 24 formed in the housing 2 have sufficient lengths to seal the channel portions of the slide bars 3 and 4, respectively, when they reciprocate.
In FIGS. 4 and 5, the first slide bar 3 is shown to be in such a state that filtration is possible, namely, the through-channel 11 in the housing 2 is in communication with the filter portion of the first slide bar 3, and the second slide bar 4 is shown to be in a such that state the screen 9 can be cleaned or replaced, namely, the filter portion of the second slide bar 4 is exposed to the outside of the housing 2. To operate the extruder, the screen changer 1 installed in it is first adapted to have both slide bars 3 and 4 positioned in the same manner as the first slide bar 3 is shown in FIG. 4 (i.e., they are positioned in such a state that filtration is possible) and, thereafter, a molten resin material is filtered.
Stated more specifically, the molten material being delivered from the extruder enters the housing 2 through the inflow channel 11a and separates in two flows, which are filtered in the filter portions provided in the two slide bars 3 and 4 and thereafter meet in the outflow channel 11b to be discharged from the housing 2. As the extruder is operated continuously, the rejected contaminants collect on the sides of the screens 9 which face the upstream compartments 31 and 41 and the resistance to the flow of the molten material increases so much as to lower the operating efficiency of the extruder. Hence, it becomes necessary to reduce the flow resistance of the molten material by cleaning the screens 9 or replacing them with new ones. In order to perform the necessary cleaning or replacing step without shutting down the extruder, filtration is accomplished with only one of the two screens while the other screen is cleaned or replaced and, thereafter, the cleaned or substituted screen is used for filtration while the first mentioned screen is cleaned or replaced.
The slide bar 3 or 4 in which the screen 9 is to be cleaned or replaced is pushed from the state of the first slide bar 3 (see FIG. 4) to the state of the second slide bar 4 (also see FIG. 4) by means of the hydraulic cylinder 5 or 6 and, after the screen 9 is cleaned or replaced, the slide bar is pulled back to its initial state (i.e., the state of the first slide bar 3). This procedure is successively followed for the two slide bars 3 and 4 one at a time.
When the slide bars 3 and 4 are to be pushed in the process of cleaning or replacing the screens 9, either slide bar is pushed in one action. When they are to be pulled back, either slide bar makes a temporary stop so that its channel and filter portions are filled with the molten material to eliminate air and only thereafter is either slide bar pulled back to its initial state (i.e., where filtration is possible). The reason for adopting this procedure is as follows. When the screen 9 is cleaned or replaced with the filter portion being exposed to the outside of the housing 2, the channel and filter portions of slide bar 3 (or 4) are totally or partially emptied of the molten material and if such "empty" slide bar is pulled back to the filtrable state in one action, the air in the empty space is discharged as part of the molten material to be filtered and, when the air containing molten material is subsequently extruded through the die, the air will expand rapidly and produce broken strands. To prevent this problem, the entrapped air is eliminated before either slide bar is pulled back to the initial filtrable state. To insure that the channel and filter portions of both slide bars 3 and 4 are completely filled with the molten material, those slide bars are designed to have additional features as described below.
FIGS. 6 and 7 show the state where the second slide bar 4 has been pulled back to a position intermediate between the filtrable state and the cleanable or replaceable state such that the channel and filter portions of said second slide bar 4 are sealed and isolated within the through-hole 24 in the housing 2, and as shown, a material injection channel 43 which communicates the upstream compartment 41 of said filter portion with the inflow channel 11a in the housing, a first air withdrawing channel 44 which communicates the topmost part of the entrance end of the upstream compartment 41 to the outside of the housing 2, and a third air withdrawing channel 45 which communicates the topmost part of the exit end of the downstream compartment 42 of said filter portion to the outside of the housing 2 are provided axially in said intermediate position along the outer surface of the second slide bar 4. The first slide bar 3 is similarly provided with a material injection channel 33, a first air withdrawing channel 34 and a third air withdrawing channel 35.
As shown with the first slide bar 3 in FIG. 4, the tips of the material injection channel 33 (34), the first air withdrawing channel 34 (44) and the third air withdrawing channel 35 (45) are sealed within the through-hole 23 (24) in the housing 2 when the respective slide bar is in the filtrable state. When the second slide bar 4 is in the intermediate position shown in FIGS. 6 and 7, its channel and filter portions are emptied of air and filled with the molten material in the following manner.
FIGS. 8 to 11 show the sequence of events that occur in that part of the screen changer which is shown in FIG. 5. FIG. 8 shows the state of the channel and filter portions of the second slide bar 4 before it is pulled back to the intermediate position. As shown, the upstream compartment 41 of the slide bar 4 is empty or filled with nothing but air a1. The downstream compartment 42 has a molten material m2 remaining in the lower part (which is sometimes completely absent as a result of cleaning) and has an air portion a2 present in the upper part.
In FIG. 9, a molten material m1 flows gradually from the inflow channel 11a into the upstream compartment 41 via the material injection channel 43 (not shown) provided in an area closer to the viewer and as the upstream compartment 41 is progressively filled with the molten material m1, air a1 is pushed for displacement to the outside of the housing 2 via the first air withdrawing channel 44. The molten material m1 which continues to flow passes through the screen 9 and the breaker plate 10 to enter the downstream compartment 42 and as the latter is progressively filled with the molten material m1, air a2 is pushed to the outside of the housing 2 via the third air withdrawing channel 45.
In FIG. 10, the upstream compartment 41 is completely filled with the molten material m1 and the air a2 remaining at the corners in the topmost part of the downstream compartment 42 (see FIG. 9) is pushed out of said compartment by means of the viscosity of the molten materials m1 and m2 which completely fill said downstream compartment 42. The upstream compartment 41 and the downstream compartment 42 are known to be filled with the molten materials m1 and m2 by when molten material is discharged through the tips of the first and third air withdrawing channels 44 and 45 which communicate with the outside of the housing 2. In FIG. 11, the upstream and downstream compartments 41 and 42 are filled with the molten materials m1 and m2; that is, the channel and filter portions of the second slide bar 4 are completely filled with the molten material; thus, the second slide bar 4 has been pulled back to the filtrable state where the filter portion communicates with the inflow channel 11a and the outflow channel 11b in the housing 2. The above-described procedure allows the screens 9 to be cleaned or replaced without allowing air to be entrapped in the molten material.
Being constructed in the way described above, the conventional screen changer has suffered from various problems. First, if the molten material to be supplied has a viscosity lower than 3000 poises, the air at the corners in the topmost part of the downstream compartment shown in FIG. 9 is not completely displaced by the pushing action of the molten material but will often remain in that downstream compartment. Such residual air is sensitive to the slightest change in the flow conditions of the molten material and will abruptly be discharged as part of the molten material.
If a molten material having viscosity lower than 1000 poises flows into the upstream compartment via the material injection channel, the air in an empty space in the upstream compartment is entrapped by the molten material flowing into that space and may form tiny bubbles that are dispersed in the molten material. This means that even if the operator verifies that the channel and filter portions of either slide bar have been filled with the molten material (i.e., air has been displaced), the residual air will be discharged as part of the molten material after the slide bar is pulled back to the filtrable state. If the molten material containing air is extruded as a strand through a die, the transition from the high pressure in the extruder to a lower atmosphere occurs so rapidly that the hot and pressurized air in the molten material will burst as a result of sudden expansion, thereby causing broken strands to emerge from the die. A continuous strand emerging from the die is taken up by a granulator for granulation but if it breaks in the middle, the subsequent portion is discontinuous from the granulator and cannot be taken by the granulator. The subsequent portion of the broken strand not only fails to be taken up by the granulator, but broken strands will keep emerging from the die and, having nowhere to go, they will collect progressively until normal operation of the extruder is prevented.
If broken strands occur, the trouble must be quickly located so that it can be remedied promptly. However, this trouble-shooting procedure is very difficult to accomplish mechanically or automatically without a large amount of labor. What is more, the molten material is oxidized upon contact with air and the resulting "burn mark" will deteriorate the quality of the granulated product. Contamination of the molten material by air will potentially cause such inconveniences to occur every time the screen changer is used for screen cleaning or replacement, and this causes a reduction in the operating efficiency of the extruder and a reduction in the the yield of the product.